Technical Field
The disclosure relates to processing methods for making stable and reliable optical stacks comprising transparent conductive films of silver nanowires and optically clear adhesives (OCA).
Description of the Related Art
Transparent conductors refer to thin conductive films coated on high-transmittance surfaces or substrates. Transparent conductors may be manufactured to have surface conductivity while maintaining reasonable optical transparency. Such surface conducting transparent conductors are widely used as transparent electrodes in flat liquid crystal displays, touch panels, electroluminescent devices, and thin film photovoltaic cells; as anti-static layers; and as electromagnetic wave shielding layers.
Currently, vacuum deposited metal oxides, such as indium tin oxide (ITO), are the industry standard materials for providing optical transparency and electrical conductivity to dielectric surfaces such as glass and polymeric films. However, metal oxide films are fragile and prone to damage during bending or other physical stresses. They also require elevated deposition temperatures and/or high annealing temperatures to achieve high conductivity levels. For certain substrates that are prone to adsorbing moisture, such as plastic and organic substrates (e.g., polycarbonates), it becomes problematic for a metal oxide film to adhere properly. Applications of metal oxide films on flexible substrates are therefore severely limited. In addition, vacuum deposition is a costly process and requires specialized equipment. Moreover, the process of vacuum deposition is not conducive to forming patterns and circuits. This typically results in the need for expensive patterning processes such as photolithography.
In recent years there is a trend to replace current industry standard transparent conductive ITO films in flat panel displays with a composite material of metal nanostructures (e.g., silver nanowires) embedded in an insulating matrix. Typically, a transparent conductive film is formed by first coating on a substrate an ink composition including silver nanowires and a binder. The binder provides the insulating matrix. The resulting transparent conductive film has a sheet resistance comparable or superior to that of the ITO films.
Nanostructure-based coating technologies are particularly suited for printed electronics. Using a solution-based format, printed electronic technology makes it possible to produce robust electronics on large-area, flexible substrates. See U.S. Pat. No. 8,049,333, in the name of Cambrios Technologies Corporation, which is hereby incorporated by reference in its entirety. The solution-based format for forming nanostructure-based thin film is also compatible with existing coating and lamination techniques. Thus, additional thin films of overcoat, undercoat, adhesive layer, and/or protective layer can be integrated into a high through-put process for forming optical stacks that include nanostructure-based transparent conductors.
Although generally considered as a noble metal, silver can be sensitive to corrosion under specific circumstances. One result of silver corrosion is a loss of conductivity either locally or uniformly, which manifests as drifts in sheet resistance of the transparent conductive film, leading to an unreliable performance. Accordingly, there remains a need in the art to provide reliable and stable optical stacks incorporating nanostructure-based transparent conductor.